Zirconium—cerium-based mixed oxides as promoters or catalyst supports are generally known to perform better than zirconia or ceria alone.
A number of processes have been proposed for the preparation of such zirconium—cerium-based oxides: for example, a sol process which comprises mixing zirconium sol and cerium sol, adding alkali to the mixed sols to form precipitates, and calcining the precipitates [Japan Kokai Tokkyo Koho Hei 6-279,027 (1994)]; a process which comprises heating the particles of zirconium hydroxide with cerium sol in the presence of nitric acid to effect dissolution and reprecipitation, adding alkali, allowing the mixture to react further, and calcining the product followed by pulverising [Japan Kokai Tokkyo Koho Hei 10-194,742 (1998)]; and a process which comprises adding oxalic acid to an aqueous solution of acidic salts of zirconium and cerium to precipitate zirconium—cerium oxalate, thermally decomposing the resulting oxalate in a non-oxidising atmosphere and then heating the decomposed product in an oxidising atmosphere [Japan Kokai Tokkyo Koho Hei 11-165,067 (1999)].
The sol process has an advantage of yielding zirconium—cerium-based mixed oxides with a relatively large specific surface area, but it faces problems such as the necessity for advance preparation of sol which is a disadvantage in respect to cost and the dried and calcined product being hard and easy to aggregate.
The process which involves heating in the presence of nitric acid is advantageous in that the use of the particles of zirconium hydroxide keeps the product from aggregating and solidifying. However, a single crystal phase becomes difficult to obtain and the range of composition which can be prepared becomes narrow when the amount of CeO2 is increased relative to that of ZrO2.
The process involving thermal decomposition of the oxalate has an advantage of readily yielding zirconium—cerium-based mixed oxides of single crystal phase because of the formation of the oxalate by coprecipitation. This process, however, requires a heat treatment at elevated temperatures in order to conduct the thermal decomposition sufficiently which causes problems such as a decrease in specific surface area and an increase in calcining cost.
Japan Kokai Tokkyo Koho Hei 10-212,122 (1998) proposes fine zirconia-ceria particles and a process for preparing the same: the ZrO2 particles by themselves are primary particles with a BET specific surface area of 40-200 m2/g, an average particle diameter determined by electron microscopy of 0.1 μm or less and a ratio of the average particle diameter determined by electron microscopy to the average particle diameter determined from BET specific surface area of 0.9 or more and the molar ratio CeO2/ZrO2 is 5/95-60/40; the fine zirconia-ceria particles show high oxygen supply efficiency in exhaust gas, adsorb or release oxygen well at low exhaust gas temperatures and can be mixed homogeneously with three-way catalysts for purifying automotive exhaust gas.
The process of preparation described in the specification of the aforementioned patent, however, requires a long period of boiling, occasionally extending over several hundred hours, to effect the hydrolysis of an aqueous solution of zirconium salt to hydrated zirconia sol with an average particle diameter of 0.1 μm or less. Moreover, the hydrated zirconia sol thus obtained is too fine to permit the application of usual industrial procedures for solid-liquid separation such as filtration under reduced pressure, filtration under pressure and centrifugal separation. In consequence, a troublesome procedure such as sedimentation and separation of supernatant liquid would be required in the steps for filtration and water washing of the hydrated zirconia sol. An operation such as aggregation, if carried out during these steps, causes a sharp decrease in specific surface area. Furthermore, the specific surface area tends to diminish rapidly when calcination is effected at an elevated temperature close to the working temperature.
A process for efficiently preparing thermally durable zirconium—cerium-based mixed oxides is proposed in Japan Kokai Tokkyo Koho Hei 11-292,539 (1999): the process comprises dispersing basic zirconium sulphate in water, mixing the dispersion with a solution containing cerium ions such as a solution of cerium nitrate, adding alkali to the mixture to yield hydroxides, effecting solid-liquid separation of the hydroxides and calcining the hydroxides.
This process utilises basic zirconium sulphate with an average particle diameter of 0.5˜20 μm and hence has an advantage of yielding zirconium—cerium-based mixed oxides with a relatively large specific surface area at elevated temperatures, for example, 100 m2/g or more at 400° C. and 30 m2/g or more at 1,000° C. However, basic zirconium sulphate with an average particle diameter of 0.5˜20 μm must be prepared in advance, which requires an extra manufacturing step with a concomitant rise in cost. Besides, zirconium—cerium-based mixed oxides of single crystal phase become difficult to obtain as the addition of ceria and a third component oxide increases. Hence, the desired performance becomes difficult to obtain and, as a result, the performance of the mixed oxides as a catalyst/promoter deteriorates.
A further process of coprecipitation by means of ammonia or ammonium carbonate or the like, starting from a mixed solution of zirconium nitrate and cerium nitrate is also known (Japan Kokai Tokkyo Koho Hei 9-278444). However, the precipitate obtained by this process is a bulky mixed hydroxide in the form of a gel with a high water content; therefore productivity is poor and can hardly be regarded as suited to industrial scale production. In addition it states that it is necessary to have the cerium salt in the tetravalent state, which is difficult to control and is not necessary in the present invention.
Thus, a filtration process is essential in order to remove impurities from the gel precipitate, and the bulkiness of the precipitate means that unit treatment speed is also invariably slow. Moreover, the high water content increases the energy needed in order to convert it to the oxides.
The use of sulphate as a precipitation modifier has been used in Japan Kokai Tokkyo Koho Hei 8-34613 and 8-34614 (1996). These, together with Japan Kokai Tokkyo Koho Hei 8-34612 (1996), describe the production of yttria-doped zirconias. In 8-34612 and 8-34613 hydrogen peroxide is added to the zirconium salt as a masking agent in order to bring the pH's of precipitation of the zirconium and yttrium salts closer together to allow homogeneous precipitation. Sulphate is added to modify the precipitation in 8-34613 and 8-34614. In the former urea is used as the precipitant and in the latter ammonia, but in three of these cases the use of alkali metal hydroxides is proscribed. Furthermore, the use of ammonia is to be deprecated because of its adverse environmental effects.
Accordingly, an object of this invention is to provide a process, easily practicable on a commercial scale, for preparing zirconium—cerium-based mixed oxides which not only possess good thermal stability at elevated temperatures but are also highly homogeneous in their crystal phase.